1. Field
The following description relates to a seismic imaging technology for imaging a subsurface structure by processing measured data reflected from the subsurface structure after a wave from a source wave has been propagated to the subsurface structure.
2. Description of the Related Art
Geophysicists study subsurface structures using information recorded at the surface in various ways, one of which is a full waveform inversion. However, recovering an accurate velocity model from field data or even synthetic data can be difficult due to local minima in the objective functions and the absence of low-frequency components in the data. These difficulties lead to the use of a two-step inversion process. In the first step, a largescale velocity model is recovered, and in the second step, inversion is performed to improve the resolution of the velocity model. There are several methods that can be used to obtain a reliable macro-velocity model, such as traveltime tomography disclosed in “Luo, Y. and Schuster, G. T. 1991. Wave-equation traveltime inversion. Geophysics 56, 645-653”, and inversion algorithms in the Laplace domain disclosed in “Shin, C. and Cha, Y. H.2008. Waveform inversion in the Laplace domain. Geophysical Journal International 173, 922-931. 2008.” Traveltime tomography reconstructs the subsurface velocity model by minimizing the residuals between the modeled and observed traveltimes. However, its penetration depth can be prohibitively shallow, and the method relies on the arrival times chosen. The macro-velocity model can also be estimated by an inversion in the Laplace domain. The Laplace-domain inversion scheme has been shown to be successful in recovering reliable macro-velocity models from field data containing a small amount of low-frequency components. However, the Laplace-domain inversion suffers from noise appearing before the first arrival. Muting is therefore mandatory in Laplace-domain inversion.